1. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor device using a semiconductor film having a crystalline structure. In particular, the invention relates to a crystallization technology for forming a semiconductor film having a crystalline structure by irradiating a semiconductor film with a laser beam.
2. Description of the Related Art
In recent years, a laser crystallization technique for forming a semiconductor film having a crystalline structure (hereinafter, a crystalline semiconductor film) by irradiating an amorphous semiconductor film which is formed over a glass substrate with a laser beam has been researched well. A crystalline semiconductor film is used because of high mobility as compared with an amorphous semiconductor film. Crystalline semiconductor films over a glass substrate are used, for example, for an active matrix liquid crystal display device or an organic EL display device, in each of which thin film transistors for a pixel portion or for a pixel portion and a driver circuit are formed over one glass substrate.
As a crystallization method, a thermal annealing method using an annealing furnace, a rapid thermal annealing method (RTA method), a laser annealing method (a crystallization method by laser irradiation), or the like can be given. In a case of using a solid phase growth method like a thermal annealing method, high-temperature treatment is performed at 600° C. or more is performed; therefore, an expensive quartz substrate that can withstand the high temperature is needed, which increases a manufacturing cost. On the other hand, when a laser beam is used for crystallization, crystallization can be performed by making only a semiconductor film absorb heat without significantly increasing the temperature of a substrate. Therefore, a material having a low melting point such as glass or plastic can be used for a substrate.
As one of laser annealing methods, there is a crystallization method using an excimer laser, which is a pulsed laser. The wavelength of an excimer laser is in an ultraviolet region, and absorptance with respect to silicon is high. Therefore, when an excimer laser is used, most of the laser beam can be absorbed into silicon. For example, in the case performing excimer laser annealing, a rectangular laser spot of approximately 10 mm×30 mm which is formed with light emitted from an excimer laser is shaped into a linear beam spot of several hundreds of μm in width and greater than or equal to 300 mm in length by using an optical system. The linearized beam spot is scanned with respect to the silicon film over the substrate to crystallize the silicon film. In this specification, a rectangular or elliptical shape having a high aspect ratio (of 10 or more) is referred to as a linear shape.
As another annealing method, there are crystallization methods using a continuous-wave laser (hereinafter, referred to as a CW laser) and a pulsed laser having a repetition rate as high as 10 MHz or more. Also in such laser annealing using a laser, a beam emitted from a laser is shaped into a linear beam spot, and the linear beam spot is scanned to irradiate the silicon film, thereby crystallizing the silicon film. When a CW laser or a pulsed laser having a high repetition rate is used, a silicon film can be completely melted and crystallized; thus, a crystalline silicon film having a crystal with a significantly large grain size (hereinafter referred to as, a large grain crystal) can be formed as compared with the case of excimer laser annealing (for example, see Reference 1: Japanese Published Patent Application No. 2005-191546). That is because a solid-liquid interface can be scanned with the linear beam spot in the laser annealing using a CW laser or the like, thereby making a crystal grow laterally while crystallization is performed by incidental core nucleation caused between the silicon film and its base interface in excimer laser annealing.
When this large grain crystal is used for a channel formation region of a thin film transistor, few crystal grain boundaries are included in the channel direction; therefore, an energy barrier against carriers such as electrons or holes gets lower. Consequently, it is possible to manufacture a thin film transistor having mobility of approximately 100 cm2/Vs.